After traveling for 23 hours straight, we tumbled out of a tiny skiff onto a sandbar on an Amazon tributary called the Tapajos River. About 45 minutes later we were awoken by the thuds of dropping mangoes, and some persistent roosters -- all around us, human figures began emerging from the dark jungle -- most of them in spandex.

Some go to Brazil for the beaches, others the cuisine or the nightlife, we came for the pain. Specifically to document the soul-destroying slog called the Jungle Marathon. One of a few ultra marathons in the world this one curled through the jungle for 137 torturous miles over seven days.

And athletes actually pay for the experience of mind-bending agony in this jungle paradise -- the biggest threat the night we landed: giant ants and those falling mangoes.

It was the start of a crazy week-long boondoggle covering what has to be one of the craziest races on earth – the wisdom of which I doubted during several moments of the trip.

We were 200 miles from a Brazilian town you'd never heard of called Santarem, a place where a tableau of human emotions would unfold, where egos would be obliterated and where technology didn't exist.

At the race's starting point was marathon director Shirley Thompson, 55, who has the build of a hummingbird but the mentality of a drill sergeant. She has been organizing this marathon for the past seven years.

Thompson greeted me, "Nightline" producer Eric Johnson and our intrepid cameraman Kenny Chow from her "command ship" -- a jalopy of a ship about the size and shape of a U-Haul.

"Let me see, I will be surprised if, well today I hope everybody will get through, but I would say that probably 65 percent," she said. "We'll have at least a 35 percent drop out this year."

That's her business model. You see if too many people survive this killer race, it'd be too easy and folks don't want to pay for easy. But we would have paid for easy.

We had hauled ourselves to one of the most remote spots on earth to follow a former Miss Florida turned veterinarian named Dr. Juli Goldstein. Goldstein has competed in about 20 marathons, and with her bright pink compression socks, matching pink shirt, and dazzling smile, she looked the part of a beauty queen -- at the start at least. She said was running the Jungle Marathon on behalf of her foundation, Wag-Strong, which she founded after her dog Strider died of cancer last year.

As the only American in the race this year and one of a handful of women competing, Goldstein told me, "I'm still girly, but I have my lip gloss with me just to have something that is a little treasure. You start to appreciate very basic things when you are doing this."

Indeed, the toilets are often holes in the ground or just open forest and everything you would need to survive a week in the jungle -- from antifungal spray to a jungle hammock to food -- is carried on your blistered back -- Goldstein's pack started out at nearly 40 pounds.

Shocking to those sane of mind, these types of extreme races are booming. In 2006 an estimated 725,000 people took part in such extreme endurance races, in 2010 that number soared to over 1.3 million, an 85 percent spike.

After Shirley Thompson begins the countdown, the race starts slowly with 60 runners slinking into the jungle.

Trying to follow them becomes nearly impossible.

Our guide leads us on a wild four-hour trek through the forest. We ford a stream (bang, my flip camera is soaked and dies) and walk. And walk and walk.